Our main objective is to produce and detect chromosomal inversions and other specific chromosomal aberrations in mice by exposing sperm and spermatids to mutagens. We identify the relationship of each aberration with genes on the respective chromosomes as well as identifying the chromosomal bands involved in the breakpoints. Each aberration is maintained in a homozygous stock, or if homozygous lethal, balanced with a genetically marked chromosomal homologue. Our objective is to use these inversions as tools for studies in mammalian mutagenesis, cytogenetics, linkage, meiotic and mitotic recombination, and dominant effects of recessive lethals and detrimentals. In addition, we are studying the aberrations themselves to determine their effect on fertility and viability. Over 30 inversions and 20 reciprocal translocations have been induced. Of these, 26 inversions and 4 translocations are being propagated and studied. Using the balanced lethal test system involving one inversion for validation, we have produced, detected and retained for study four recessive lethals. Inversions are being combined with each other and with Robertsonian chromosomes to increase the power of the system.